1. Field of the Invention
This invention relates to gas turbine engines, and more particularly, to coatings applied to airfoils in the turbine portion of the gas turbine engine.
2. Discussion of Prior Art
The current coatings used on airfoils exposed to the hot gases of combustion in gas turbine engines for both environmental protection and as bond coats in thermal barrier coating (TBC) systems include both diffusion aluminides and MCrAlY(X) coatings. These coatings are applied over substrate materials, typically nickel-base superalloys, to provide protection against oxidation and corrosion attack. These coatings are formed on the substrate in a number of different ways. For example, a nickel aluminide (NiAl) layer may be grown as an outer coat on a nickel base superalloy by simply exposing the substrate to an aluminum rich environment at elevated temperatures. The aluminum diffuses into the substrate and combines with the nickel to form an outer surface of NiAl. A platinum modified nickel aluminide coating can be formed by first electroplating platinum to a predetermined thickness over the nickel-based substrate. Exposure of the platinum-plated substrate to an aluminum-rich environment at elevated temperatures causes the growth of an outer region of NiAl containing platinum in solid solution. In the presence of excess aluminum PtAl2 phases may precipitate in the NiAl matrix as the aluminum diffuses into and reacts with the nickel and platinum. Of course, an overlay of MCrAlY where M is an element selected from the group consisting of Ni and Co and combinations thereof may be applied to the substrate as a bond coat or as an environmental coating by any known technique. When applied as bond coats in thermal barrier systems, an additional thermally resistant coating such as yttria-stabilized zirconia (YSZ) is applied over top of the coating. However, as the increased demands for engine performance elevate the engine operating temperatures and/or the engine life requirements, improvements in the performance of coatings when used as environmental coatings or as bond coatings are needed over and above the capabilities of these existing coatings. Because of these demands, a coating that can be used for environmental protection or as a bond coat capable of withstanding higher operating temperatures or operating for a longer period of time before requiring removal for repair, or both, is required.
The present invention is directed to an improved coating for use on an airfoil in the turbine portion of a gas turbine engine. In its broadest embodiment, the coating is for application over a nickel base superalloy substrate requiring environmental protection and which is subjected to elevated temperatures. The coating is comprised of a diffusion aluminide coating selected from the group consisting of NiAl and PtAl. As used herein, unless otherwise specified, the term platinum aluminide or PtAl refers to a platinum-modified NiAl in which the NiAl includes platinum in solid solution and in which PtAlx phases may be present as precipitates in the matrix. This platinum aluminide may be subsequently modified as discussed herein. This coating is applied by two distinct diffusion aluminiding cycles resulting in two distinct regions. One region of the coating is a diffusion aluminide having at least one element selected from the group consisting of hafnium (Hf), zirconium (Zr), yttrium (Y) and silicon (Si), and combinations of these elements. The second region of the diffusion aluminide coating adjacent to the first region is substantially free of any of the elements selected from the group consisting of Hf, Zr, Y and Si, except as occurs as a result of natural diffusion processes during subsequent high temperature exposures. Airfoils having coatings with such oxygen-active elements as Hf, Si, Y, and Zr and combinations thereof exhibit longer life and are capable of withstanding higher temperatures. When used for environmental protection, these coatings are expected to exhibit less corrosion and oxidation at higher temperatures than either the prior art aluminides or MCrAlY coatings. When used as a bond coat underlying a thermal barrier coating, the bond coat of the present invention provides an advantage of improved damage resistance in terms of lower spallation values for comparable times as compared to a standard aluminide baseline. This improvement translates into longer life.